Add / Remove a dimension

Adding or removing a dimension on a given combinatorial maps effectively corresponds, respectively, to adding or removing a beta function. In the case of decreasing the dimension, this operation can result in two disjoint dart set in the same map.

Because the current implementation only covers 2D combinatiorial maps, this operation is not implemented. When 3D maps are implemented, it would be possible to implement this using the From trait provided by the Rust language.